1. Field of the Art
The invention relates to a device for the production of pipes composed of a wear resistant inner pipe and a shock and impact resistant outer pipe, with the inner pipe being prefabricated and the outer pipe being formed from a previously bent metal starting sheet which is shaped by bending it around the inner pipe and welding together its abutting longitudinal edges.
2. State of the Art
A device for producing pipes composed of a prefabricated inner pipe and an outer pipe formed by bending it around the inner pipe and welding together its abutting longitudinal edges is disclosed in U.S. Pat. No. 2,373,163. In the prior art device, four rollers engage the starting sheet, two in the region where it lies and two diametrally opposite one another in the region defined by the horizontal diameter of the starting sheet. All rollers are mounted so as to be pivotal directly or indirectly. While the starting sheet is being placed around the inner pipe, they roll along the starting sheet, with the respective diametrally oppositely disposed rollers moving into the region of the abutting longitudinal edges. The prior art device is expensive. Because of the use of rollers, it does not permit optimal application of pressure to the starting sheet. In particular, it has been found to be a drawback that the tools, here in the form of rollers, move along the circumference of the outer sheet. Consequently, pressure is applied radially successively from different directions. The distance between the lower and the upper rollers becomes greater. The danger exists that, particularly in the region between the rollers, contact between the inner pipe and the outer pipe would not be proper.
Another device for producing pipes is disclosed in DE-OS U.S. Pat. No. 3,226,776. The device disclosed in detail in this patent application (published without examination) is used to shape the metal starting sheet of the outer pipe, starting with a coil, in the form of a spiral around the completely shaped and welded inner pipe. Thereafter, the edges of the turns at the exterior surface of the spiral are welded together. The spiral shape results in a long weld seam; a complicated and expensive device is required to produce the pipe. In the method additionally disclosed in this patent application, a previously bent metal starting sheet for the outer pipe is shaped around the inner pipe which has been welded together along the longitudinal seam. Thereafter, the abutting longitudinal edges of the starting sheet are welded together. However, it is not considered to be possible in this method to realize sufficient engagement of the starting sheet with the pipe jacket of the inner pipe, particularly at the ends of the pipe; and this, although the increase in volume of the material of the inner pipe during hardening is to be exploited.